The Electoral Commission is investigating whether he breached campaign finance rules in the referendum.

Brexit campaign donor Arron Banks has insisted the vote to quit the EU was not “funded by the Russians” after the elections watchdog said it was investigating whether he breached campaign finance rules in the referendum.

Brexit campaign donor Arron Banks has insisted the vote to quit the EU was not “funded by the Russians” after the elections watchdog said it was investigating whether he breached campaign finance rules in the referendum.

The Electoral Commission said it was looking into whether Mr Banks was the “true source” of three loans worth £6 million on non-commercial terms to the Leave.EU campaign he chairs.

It is also probing whether Better for the Country Limited (BFTCL), a company that lists Mr Banks as a director and has its registered office at the same address as Leave.EU, was acting as an “agent” when it donated £2.3 million to five registered campaigners.

Labour MP Ben Bradshaw, who previously raised concerns about openDemocracy reports on the role of “dark money” in the referendum and Mr Banks’s “real wealth”, linked the announcement to the US investigation into Russia and President Donald Trump.

At last! @ElectoralCommission confirms it has opened an investigation into Arron Banks, the main financial backer of the Leave campaign.

Mr Bradshaw asked Theresa May at Prime Minister’s Questions: “With today’s news that the Electoral Commission is investigating Arron Banks, the main financial backer of Brexit, along with the significant British connections being uncovered by the American Justice Department’s special counsel Robert Mueller, investigating Russian interference in the US presidential election, will you assure me that the UK Government and all its agencies are co-operating fully with the Mueller investigation or will do so if asked?”

Mrs May replied: “We take very seriously issues of Russian intervention or Russian attempts to intervene in electoral processes or in the democratic processes of any country and we would do so for any other states that were involved in trying to intervene in elections.

“We do of course work closely with our United States partners and I can assure you that as part of that relationship we do co-operate with them when required.”

Theresa May speaks during Prime Minister's Questions in the House of Commons (PA)

Announcing the investigation, Bob Posner, the commission’s director of political finance and regulation, and legal counsel, said: “Interest in the funding of the EU referendum campaigns remains widespread.

“Questions over the legitimacy of funding provided to campaigners at the referendum risks causing harm to voters’ confidence.

“It is therefore in the public interest that the Electoral Commission seeks to ascertain whether or not impermissible donations were given to referendum campaigners and if any other related offences have taken place.”

Mr Banks said his only involvement with Russia was a “boozy six-hour lunch” with an ambassador which he has written about in a book.

He said in a statement: “The Leave.EU campaign was funded by myself, Peter Hargreaves and the general public.

“The Guardian allegations of Brexit being funded by the Russians and propagated by Ben Bradshaw are complete bollocks.

“My sole involvement with the ‘Russians’ was a boozy six-hour lunch with the ambassador where we drank the place dry (they have some cracking vodka and brandy).

“Turning to the investigation, we believe that a judge-led inquiry reporting to Parliament that investigates the main campaign groups, Vote Leave, Remain, and Leave.EU would be the best way to clear this nonsense up once and for all.

“The Remain ‘Electoral Commission’ isn’t up to the job and consists of political place men from all main parties.”

Among the recipients of donations from BFTCL was the Grassroots Out campaign which was supported by some Tory MPs, including Brexit Secretary David Davis.

BFTCL also made donations between March and June 2016 to Trade Unionists Against the EU, Veterans for Britain, WAGTV Limited, and Ukip, led at the time by Nigel Farage.